The largest single application of lead-acid storage batteries is for the starting, lighting, and ignition of automobiles, trucks and buses. These batteries are charged automatically from a generator driven by the engine while it is running and they supply power for the lights while the engine is shut-down and for ignition and cranking when the engine is started. Lead-acid storage batteries are also widely used in aircraft and boats with virtually unlimited applications also existing in non-motive situations.
Lead-acid batteries contain a series of lead-acid cells, each including a positive plate containing positive, active material, such as lead dioxide, and a negative plate containing negative, active material, such as sponge lead immersed in an electrolyte solution, typically dilute sulfuric acid. The respective positive and negative plates are connected in parallel with the power or current output of a cell being determined by the number and size of the plates. The open circuit potential developed between each pair of positive and negative plates is about two volts. Since the plates are connected in parallel, the combined potential for each cell will be also about two volts regardless of the number of plates utilized in the cell. One or more cells are then serially connected to provide a battery of desired voltage. Common low voltage batteries are 6 volt batteries having three serially connected cells and 24 volt batteries with 12 serially connected cells.
The positive and negative plates are usually oriented vertically in a horizontal stacked relationship. As a result of this vertical orientation, electrolyte stratification commonly occurs vertically along the plate surfaces. This results in diminishing of battery performance. Some attempts have been made to prevent electrolyte stratification, such as stirring of the electrolyte by various mixing systems. These mixing systems are not only cumbersome, but are expensive and subject to failure during the life of a particular battery.
Another problem with lead-acid batteries is their limited lifetime due to shedding of the active materials from the positive and negative plates. Pasted plate lead-acid batteries are by far the most common type of lead-acid battery. Typically, a paste of lead oxide is applied to the surfaces of the positive and negative grids. When an initial electric charge is applied to the plates, the lead oxide paste on the positive grid is oxidized to lead dioxide while the lead oxide on the negative plate is reduced. During continued operation of the lead-acid battery, shedding or flaking of the deposited lead paste occurs. The flakes of material fall down between the vertically oriented plates and accumulate in a well on the battery bottom. After a period of time, sufficient flakes accumulate on the battery bottom to short circuit the negative and positive grids resulting in a dead battery cell and shortened battery life.
In the past, lead-acid batteries have been inherently rather heavy due to the use of lead in constructing the plates. Recently, attempts have been made to produce light-weight, lead-acid batteries especially for use in aircraft, electric cars and other vehicles where weight is an important consideration. Emphasis has been placed on producing thinner plates made from lightweight materials used in place of or in combination with lead. Although the thinner, lightweight plates are beneficial in reducing battery weight, they present problems in regards to providing a cell structure which is sufficiently strong and rigid to prevent structural failure during normal use. The thinner plates allow the use of more plates for a given weight volume, thus increasing the power density.
In my co-pending application, Ser. No. 268,484, filed May 29, 1981, now U.S. Pat. No. 4,405,697, entitled "IMPROVED LEAD-ACID BATTERY," a lightweight battery is described which includes a plurality of horizontally oriented, vertically stacked alternating positive and negative monoplates or grids. Tabs are provided extending from two opposite edges of the plates or grids along the total length of the grids on both sides thereof. The negative and positive plates are stacked so that two positive tabs extend from the cell or grid stack on sides adjacent the two negative tabs. The common tabs on each side of the grid stack are welded together in parallel to form four bus bars or plates extending vertically up the cell sides. The bus bars not only greatly reduce electrical resistance in the battery cell or grid stack, but additionally provide rigidity and strengthening to the cell structure. Further, the horizontal orientation of the grids prevents the accumulation of flaked lead compounds at the battery bottom, since their downward movement is blocked by the glass mat containing the electrolyte placed between each set of positive and negative plates. Also, stratification of the electrolyte is avoided, since the electrolyte is confined and contained within the acid resistant glass mats by capillary action.
Although, the improved lead-acid battery disclosed in my co-pending application does away with active material flaking and electrolyte stratification while strengthening the battery structure, it would be desirable to provide a battery with different voltage potentials. In order to increase available voltage from my improved lead-acid battery, it is necessary to serially connect a number of cells together. This may not be desirable or convenient in providing a suitable high voltage lightweight battery, since to reach relatively high voltages, the number of serially connected cells necessary is large with a resultant undesirable increase in resistance and battery size.
It is also desirable to provide a low maintenance battery that does not require addition of water or electrolyte throughout its useful life. Sealed battery designs have been available in rolled configurations for sometime. There are also sealed lead-acid batteries having vertical plates. However, the sealed batteries are in fact an assemblage of sealed, individual cells. Since oxygen is generated during discharge, each cell must contain its own resealable vent. Furthermore, the individual cells develop different internal pressures leading to warping and buckling of the cell enclosure and of the battery casing.